1. Field of the Invention
This invention relates generally to radar systems and more particularly to constant false alarm rate doppler systems.
2. Description of the Prior Art
Electronic countermeasures, commonly referred to as jamming, presents a serious problem to doppler radar systems. The jamming energy causes saturation and sensitivity degradation in addition to causing many false alarms due to the required dwell time inherent in doppler radar systems.
It is well known in the art that the doppler frequency shift resulting from a relative velocity between radar and target may be used either in pulse, or continuous-wave radar to distinguish fixed targets from those in motion. However, the distinction of moving targets in itself is not the only nor is it the most important benefit to be gained from the extraction and employment of doppler information. It is also well known in the art that even when the return or echo from fixed targets called clutter, in orders of magnitude greater, say 20 to 50 db, than the moving target echo, doppler radars can nevertheless discern the moving target through utilization of doppler information. These advantages stem from the basic physical observation that echo signals from fixed targets will not undergo a frequency shift while the echo from a target exhibiting a relative velocity v.sub.r will be frequency shifter in an amount f.sub.d described by the relation EQU f.sub.d = 2v.sub.r /.lambda.
where .lambda. is the radar wavelength. Detection of moving targets has been reported even when the target echo has been 70 to 90 db below the clutter echo.
Observation of moving targets vis-a-vis stationary targets may be made by watching the video output in a radar receiver system on an A-scope. Observation of successive sweeps reveals that echoes from fixed targets do not vary appreciably, however, echoes from moving targets do vary predictably from sweep to sweep in amplitude at a rate related to the doppler frequency. Superposition of successive sweeps results in an A-scope display that distinguishes the moving targets. Such superposition, while effective for distinguishing moving targets on an A-scope, is not, however, suitable for PPI display.
One method for processing doppler information so as to render it suitable for PPI display is through the implementation of a delay-line canceler. The delay-line canceler in its simplest form, rejects the fixed target return and passes the returns from moving targets. In performing this function, received video information is fed to two separate channels in one of which the video signal is timed delayed by one pulse repetition period. The outputs from the two channels are then subtracted one from the other thus eliminating the constant amplitude signals of echoes received from fixed targets and preserving an uncanceled residue signal resulting from the subtraction of the pulse-to-pulse varying echoes received from moving targets. Output from the subtractor in an ideal system will only be produced by moving targets.
Historically reference to a doppler radar indicates a system in which the doppler frequency measurements are unambiguous but in which ranging measurements may or may not be ambiguous. Ambiguous range means that multiple-time-around echoes are possible while ambiguous doppler means that blind speeds fall within the compass of expected target speeds.